1. Field of the Invention
The present invention relates to an image processing apparatus in which an image is captured from an original document to obtain digital image data and the image data is then subjected to image processing to obtain image data suitably for the type of the apparatus. Examples of such an image processing apparatus include a digital printing press in which perforations are formed in a thermosensitive perforation master sheet in accordance with processed image data, a digital copying apparatus in which a latent image is formed on a light-sensitive material by electrophotography in accordance with processed image data and is transferred to a sheet of paper, and an apparatus in which an image is copied onto thermosensitive paper after image processing. In particular, the present invention relates to an image processing apparatus in which a captured image is converted to binary data to output binary information representing the captured image.
2. Description of the Related Art
When an original document containing both a completely black-and-white image such as a character or a line (hereinafter referred to as a "binary image") and an image having gradation (hereinafter referred to as a "gradation image") is processed by the above-described image processing apparatus, the best results can be obtained by classifying the density of the binary image, using a single threshold, into a maximum density and a minimum density and by carrying out density conversion for the gradation image area to maintain the consistency between the density of an input image and the density of an output image corresponding to the input image, taking into account the characteristics of input and output devices. Therefore, it is necessary to make a judgment as to whether each small area of an image contains a binary image or a gradation image so as to process the areas differently.
Conventionally, an image is divided into pixel blocks of n.times.n, and the characteristics of each block are extracted. By using the results of the characteristic extraction, it is judged whether each block is a binary image area (an area containing a binary image) or a gradation image area (an area containing a gradation image) (Japanese Patent Application Laid-Open (kokai) No. 3-153167). Hereinafter, this judgment may be referred to as "area judgment". Alternatively, the characteristics of a certain pixel are extracted by using the certain pixel and its peripheral pixels, and the area judgment for each pixel is performed based on the extracted characteristics (Japanese Patent Application Laid-Open (kokai) No. 1-227573).
In the former method, since the area judgment is performed for each block, rectangular shapes corresponding to the blocks are formed at portions where the area judgment is erroneously performed or at portions corresponding to the boundary between a binary image area and a gradation image area. In the latter method, the influence of an erroneous judgment is small. However, a considerable difference in density is produced between a portion in which the area judgment is erroneously performed and a portion in which the area judgment is properly performed, resulting in an unnatural vision.
Also, it is difficult to discriminate a thick line and a black solid portion in a binary image area from a high density portion of a picture in a gradation image area. When parameters for area judgment are adjusted such that the thick line and the black solid portion are judged as binary images, some portions of a picture image are destroyed. When the parameters for area judgment are adjusted such that the high density portion of the picture image is judged as a gradation image, the densities of the thick line and the black solid portion decrease.